A witness who qualifies as an expert because of knowledge, skill, experience, training, or education may testify as an expert if scientific, technical, or other specialized knowledge will assist the judge, jury, or other factfinder to understand the evidence or determine a contested fact issue.

But there is no rigid formula for determining whether a particular witness is qualified to testify as an expert. Generally there must be a fit between the subject matter at issue and the expert witness’s familiarity with the subject matter. And this determination is made by the judge, who acts as a gatekeeper to only allow reliable expert witness testimony to be heard and considered by the judge or jury.

In California, expert witness testimony is governed by both state statutes and case law. An individual may testify as an expert witness if they have the necessary knowledge, skill, experience, training, or education that would assist the court in understanding the evidence or determining a fact in question. The determination of whether a witness qualifies as an expert is not based on a strict formula but rather on the relevance and reliability of the expert's knowledge to the subject matter of the dispute. The judge serves as a gatekeeper, assessing whether the proposed testimony is pertinent to the case and whether the expert has the appropriate qualifications. This gatekeeping role involves ensuring that the expert's methods and principles are scientifically valid and can be properly applied to the facts of the case. The judge's decision to admit or exclude expert testimony can significantly impact the proceedings, as only reliable and relevant expert evidence is allowed to be presented to the jury or considered by the judge.